1. Technical Field
The present invention relates to a gas cycle refrigerator which generates refrigeration by expanding refrigerant gas, for example, Helium, and refrigerates a substance to be cooled to a cryogenic temperature of 3 to 70K (Kelvin).
2. Background Art
In recent years, the number of superconducting magnet systems and detectors have increased which require cryogenic refrigeration within the temperature range of liquid Helium. A liquid Helium refrigeration device has many inconveniences as a cryogenic refrigerator used for the above purpose, and refrigerators of the type which vary the volume of an expansion room with the movement of a piston (including a displacer) provided in a cylinder, for example, a G-M (Gifford-McMahon) cycle refrigerator and a Stirling cycle refrigerator, and a pulse tube refrigerator which eliminates moving parts such as a piston or the like, are mainly used.
These conventional refrigerators can normally only refrigerate down to about 70K in the first stage; therefore, a multiple stage configuration of about 2 or 3 stages to improve refrigeration performance is required when refrigeration to lower temperature is needed.
One type of refrigerator varies the volume of the expansion room with the movement of the piston (displacer), expanding Helium gas to generate refrigeration. The volume of the expansion room varies with the movement of the piston inside the cylinder, charging and discharging Helium gas inside the expansion room to achieve the desired temperature.
For this reason, the opening between the cylinder and piston requires a seal preventing Helium gas from escaping the inside of the expansion room. A gas cycle refrigerator typically uses a non-lubricant type seal, for example, polytetrafluoroethylene (hereinafter described as "Teflon" (Trademark)), overcoming the problem of lubricant hardening at very low temperatures.
While a first stage refrigerator doesn't have the problem of the seal material contracting or otherwise leaking because the seal can be provided at a hot (room temperature) end of the piston which has little variation of temperature, a second stage refrigerator has various problems because the part where the seal is provided has a large variation of temperature (from room temperature to cryogenic temperature).
When there is little temperature fluctuation, a metal ring is sheathed with "Teflon", for example, a Teflon O ring, for the seal in the room temperature part. The seal to be provided in the part having thermal fluctuation requires a special metal ring with only its outer perimeter provided with "Teflon", and both the metal ring and "Teflon" are step cut because the thermal contraction rate of the metal ring and the "Teflon" are different.
There is a fear that refrigeration performance will be lowered by refrigerant gas leakage because the cut of the seal and the inside of the seal where the metal ring rubs the piston can't be completely sealed, as well as a fear that the cost of the seal will be high.
A pulse tube refrigerator which eliminates moving parts such as the piston has an advantage of not having a sealing problem. However, the application was delayed for a long time because the refrigeration performance could not be improved. Optimization for the single stage pulse tube refrigerator has been achieved recently through experimentation, and the refrigerator with high refrigeration efficiency has been put to actual applications.
However, the pulse tube refrigerator also requires multiple staging to lower the refrigeration temperature further. Multiple staging causes mutual interference between each stage part, requiring large-scale experiments for optimization. Thus, there is a problem of not being able to improve the refrigeration efficiency and the difficulty in optimization.
It is an object of the present invention to provide a gas cycle refrigerator having a multiple stage configuration and improved the refrigeration performance.